This invention relates to a method for stimulating a mucosal immune response. More particularly, this invention relates to the use of hydrogels (water swellable, cross-linked polymers) as vehicles for delivering antigens to the mucosa-associated lymphoid tissue of the gut.
Mucosal surfaces are the sites of entry of most infectious agents and hosts. Therefore, mucosal immunity is important as the first line of defense against infectious agents. It prevents attachment of pathogens to the mucosal epithelium, neutralizes viruses and bacteria toxins, and allows other aspects of the immune system to phagocytose and remove pathogens from the mucosal site. The mucosa-associated immune system functions to prevent the penetration of microbes into the internal regions of the body.
Direct application of antigens to a mucosal surface is the best way to induce a local immune response. However, this is not always possible or practical either because of the handling involved or because the toxicity of the antigens to the mucosal surface. Local immunity can also be induced by the stimulation of the common mucosal immune system (MALT), a network whereby all mucosal sites are linked to each other immunologically. When the mucosa-associated lymphoid tissue of the gut or lung is exposed to an antigen, lymphocytes migrate to all other mucosal sites and produce antibodies. A population of memory lymphocytes is also induced which provides antibodies at a later time in response to the antigen.
The greatest accumulation of lymphoid tissue in MALT and in the body is the gut-associated lymphoid tissue (GALT) located in the intestines. These lymphoid tissues contain functional T and B lymphocytes and antigen-presenting accessory cells. In contrast to the systemic lymphoid tissues of the body, the B lymphocyte population of GALT includes a significant population of cells which are committed to the synthesis of IgA class antibodies. This antibody type is not effectively induced through conventional intramuscular or subcutaneous immunization. The lymphoid tissue is separated from the lumen of the gut by a layer of epithelial cells which are interspersed with antigen-presenting accessory cells. These specialized cells actively internalize samples from the lumenal space, and pass the samples to the underlying lymphoid cells. Exposure of GALT to antigen compounds triggers the clonal expansion of specific B and T lymphocytes. The IgA committed B lymphoblasts migrate through the roesenteric lymph nodes resulting in enhanced immune responses in all mucosal sites including the intestine, lung, mouth, eye, mammary gland, and the genitourinary tract. Thus, stimulation of GALT by oral vaccines can result in the prevention of infectious diseases at a variety of mucosal surfaces.
Timely vaccination of livestock can be a critical aspect of effective farm management. Respiratory disease of viral and secondary bacterial etiology can spread rapidly through animal herds. Although stimulation of mucosal immunity can be achieved by intranasal administration or local injection into mucosal sites, such vaccination techniques typically require individual handling and restraint of each animal. Oral vaccination is a particularly cost effective way for livestock producers to vaccinate or treat a large number of animals at one time with minimal stress or labor. This is especially true when oral administration of the vaccine can be effected through ingestion by the animals during the course of feeding. Further, oral vaccines can be manufactured more cost effectively than parenterally administered vaccine formulations because of the fewer purification steps needed to generate an oral vaccine. Oral vaccination also offers the advantage of fewer side effects such as fever or other injection reactions.
In spite of the advantages deriving from use of oral vaccines, the use and efficacy of orally administered vaccines has been hampered by degradation of the vaccine as it passes through the digestive tract. Most antigenic compounds possess peptide bonds in their configuration that are readily decomposed and denatured by the gastric acidity and proteolytic enzymes of the gut. Drug delivery systems have been proposed to protect orally ingested antigenic compounds from enzymatic or hydrolytic degradation. These delivery systems involve coating the vaccine in gelatin capsules or other coatings that are not removed until passage into the intestine. Prior art also describes the use of biodegradable microspheres for delivering vaccines to the gut-associated lymphold tissues. These microspheres, being a maximum size of 10 micrometers in diameter, are directly taken up into the lymphold tissues to stimulate an immune response.
Bovine Respiratory Disease Complex costs the cattle industry in the United States over one hundred million dollars a year. Conventional vaccination strategies have not been successful in preventing the disease. Cattle are usually vaccinated at sale barns or at stock yards when they have already been stressed and exposed to a number of pathogens. A better time to vaccinate cattle is before they are sold, optimally when they are still in the pasture. This is typically not done due to the labor required to inject each animal individually. An efficient way to vaccinate the cattle in a pasture would be through feed or water. Recent studies have shown that it is possible to stimulate antibodies to P. haemolytica, the most common cause of bacterial pneumonia in cattle, by stimulating GALT in cattle with bacterial antigens.
However, bovine and other ruminant species present a greater challenge to oral vaccination because the administered antigen dose has four stomachs to traverse in route to the small intestine where GALT tissues are located. To be practical, orally dosed therapeutic agents for ruminant species must meet a number of design specifications. First, the dosage form must be easy to administer, easily swallowed and not be readily regurgitated by the animal. Secondly, in terms of release rate performance, the device must deliver a therapeutically effective dose accurately and reliably without a catastrophic failure or "dose dumping." Finally, the drug and delivery device must be cost effective.
Oral vaccines have not typically been used in cattle because the harsh environment of the rumen results in microbial digestion of any unprotected antigens. The rumen is the first of four stomachs present in ruminants. It functions by fermentation digestion by bacteria action to break down cellulose and other complex nutrients. Therefore, an oral vaccine for cattle must be protected while passing through the rumen in order to deliver an effective dose of antigen to GALT in the lower gastrointestinal tract.
In accordance with this invention, hydrogels have been found to be an effective delivery vehicle or matrix capable of protecting an antigenically active vaccine formulation while it passes through the rumen. A hydrogel is a water swellable, cross-linked polymer well known to those of ordinary skill in the art. See, for example, Dresback, U.S. Pat. No. 4,220,152, issued Sep. 2, 1980, the disclosure of which is expressly incorporated herein by reference. The hydrogel dosage forms have been found to pass without substantial hydration through the rumen to become localized in the reticulum, where they are hydrated and concomitantly release the vaccine (antigenic composition) held in the hydrogel. The released vaccine is carried into the small intestine, without loss of antigenicity, where it makes contact with GALT to induce an immune response.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.